Clean-enclosure window to protect photolithographic mask

ABSTRACT

A clean-enclosure to protect a reticle from contamination when using extreme ultraviolet (EUV) lithography is disclosed. The clean-enclosure consists of frame and a cover attached to the frame. The cover contains an exposure window comprised of a thin film of silicon. This thin film window allows EUV light to pass through to the reticle and reflect onto the photoresist layer of a semiconductor substrate with minimal transmission loss. Also, a process for forming the silicon thin film exposure window is disclosed.

FIELD OF THE INVENTION

The present invention relates to the field of photolithography. Morespecifically, the invention relates to a way of protecting aphotolithographic mask using a clean-enclosure.

BACKGROUND OF THE INVENTION

A critical step in semiconductor processing is photolithography. Theability to achieve smaller and smaller dimensions on an integratedcircuit is generally understood to be limited more by photolithographythan any other fabrication step of the semiconductor process. As theindustry heads toward forming submicron line dimensions, advancedphotolithographic capabilities become ever more critical. Various newphotolithographic techniques using smaller wavelength light sources arebeing developed, including Deep Ultra Violet Lithography, Extreme UltraViolet Lithography (EUVL), and x-ray lithography.

In typical photolithography techniques, radiation from a light source isprojected through a reticle, that is, a patterned mask, and an image ofthe pattern on the reticle is focused through a lens onto the radiationsensitive photoresist layer of a semiconductor substrate. The substratemay be a silicon wafer or other semiconductor substrate on whichintegrated circuits or micromechanical structures are fabricated.

A typical reticle comprises a patterned opaque material applied to oneside of a transparent base. The base, typically comprised of quartz, istransparent to the projected radiation. The patterned opaque material,typically chrome, is opaque to the projected radiation. In addition tothe desired pattern any defect in the reticle will also be projectedonto the photoresist layer of the semiconductor substrate. For example,if a particle is present on the reticle during exposure of thephotoresist layer, the image of the particle may be focused onto thephotoresist layer. This corresponding defect in the photoresist patternon the semiconductor substrate may cause the failure of thesemiconductor device being manufactured.

EUVL, which typically uses a light source with a wavelength on the orderof 13 nanometers (nm), is a promising technology for submicronintegrated circuit fabrication. The base of a typical reticle is nottransparent to ultra violet radiation in the extreme ultra violet (EUV)range because of the strong absorption of the base material. Therefore,a reflective reticle is used in EUVL.

Even when using reflective reticles, any reticle defects may be imagedonto the photoresist layer of the semiconductor substrate. The surfaceof the reflective reticle is very difficult to keep clean and anysemiconductor device being manufactured may fail if particles arepresent on the reticle. The images of the particles may be focused ontothe photoresist layer during exposure, leading to unacceptable defectsin the semiconductor device.

Typically, a pellicle is used to protect a reticle and to keep it clean.A pellicle is a thin, flat, transparent membrane, usually made of anorganic material. The pellicle is held by a frame and placed over thereticle. The frame of the pellicle holds it several millimeters awayfrom the patterned surface of the reticle. The pellicle keeps particlesfrom falling onto the surface of the reticle. Any particles that fallonto the pellicle will be outside the focal plane of thephotolithography system, and therefore will not focus onto thesemiconductor wafer during exposure.

FIG. 1 illustrates a prior art photolithography mask having a pellicle110 mounted on the surface of a reticle 100. Pellicle 110 forms acovered or protected area 120 over the patterned area of reticle 100.Pellicle 110 includes a frame 130 and a thin, transparent membrane 140.

Pellicle 110 is effective at reducing the likelihood that particles willmigrate onto reticle 100; however, prior art organic pellicle membranes140 cannot be used in EUVL because these pellicle membranes are nottransparent to EUV radiation. The pellicle membranes absorb anunacceptable amount of ultra violet light in the EUV range, especiallywhen using the reflective reticles required in EUVL. This is because thesource EUV radiation is absorbed twice as it makes a dual pass throughthe pellicle membrane on its reflected path to the semiconductor wafer.

There exists a need for a non-pellicle device to protect reticles and toprevent defects in semiconductor devices manufactured using EUVL. Itwould be advantageous to have a reticle-protective device that isreadily manufacturable and compatible with both EUVL and currently usedphotolithographic manufacturing techniques.

SUMMARY OF THE INVENTION

Briefly, a clean-enclosure for protecting photolithographic reticlesincludes a frame and a cover bonded to the frame. The cover has anopening sealed with a window. The window comprises a thin film materialthat is transmissive to a predetermined range of wavelengths of light.

In a further aspect of the present invention, a method of forming asilicon thin film window is also described. A silicon oxide layer isdeposited on the front side of a silicon wafer. A silicon thin film isdeposited on top of this oxide layer. An opening is etched through thebackside of the wafer to the oxide layer. The oxide layer in the openingis removed, leaving a thin film silicon window.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art photolithographic maskcomprising a reticle protected by a conventional organic pellicle.

FIG. 2 is a cross-sectional view of a photolithographic mask comprisinga reflective reticle on a moving base protected by a clean-enclosure inaccordance with a preferred embodiment.

FIG. 3 is a graph showing the dual-path transmission of extremeultraviolet (13 nm) light as a function of the thickness of a thin filmof silicon.

FIG. 4 is a top view of the approximate shape and size of the EUVradiation illumination, based on ringfield EUVL design, reflected on thereticle plane.

FIG. 5 is a top view of the FIG. 2 clean-enclosure cover and thin filmexposure window taken along a line connecting A-A′ of FIG. 2.

FIG. 6 is an enlarged cross-sectional view of the center portion of theFIG. 2 photolithographic mask showing the source radiation as it makes adual pass through the thin film exposure window on its reflected path tothe semiconductor wafer.

FIG. 7 is a cross-sectional view of a silicon wafer substrate used inthe formation of the thin film exposure window.

FIG. 8 is a cross-sectional view of the substrate of FIG. 7 after asilicon oxide layer is deposited.

FIG. 9 is a cross-sectional view of the substrate of FIG. 8 after asilicon thin film is deposited.

FIG. 10 is a cross-sectional view of the substrate of FIG. 9 after aphotoresist layer is applied.

FIG. 11 is a cross-sectional view of the substrate of FIG. 10 after athin film exposure window is etched into the silicon oxide layer.

FIG. 12 is a cross-sectional view of the substrate of FIG. 11 afterbeing wet etched.

FIG. 13 is a perspective view of the window of the FIG. 12 mask inaccordance with a preferred embodiment.

DETAILED DESCRIPTION

The invention includes a clean-enclosure which, when used as part of thephotolithographic process of manufacturing microelectronic ormicromechanical devices, reduces the number of particles that migrateonto the surface of a reticle. The invention may be used with anyphotolithographic process where particles on a reticle surface may causethe formation of defective patterns within a radiation sensitive layerof a semiconductor substrate. However, the invention is particularlyuseful for EUVL because of the strong absorption of EUV radation inprior art materials used to protect reticles. EUVL will be the contextin describing the invention, although the invention can be used toprotect reticles used in photolithography at other wavelengths,including, for example, ultra violet, deep ultra violet, and x-rayradiation.

FIG. 2 is a cross-sectional view of an EUVL mask including aclean-enclosure in accordance with a preferred embodiment. The maskincludes a reflective reticle 200 mounted on a moving reticle stage base210. Moving reticle stage base 210 moves laterally across the surface ofnonmoving reticle stage base 220 in a stepping motion such that onesection of reflective reticle 200 is exposed to the stationary EUVsource with each step. Clean-enclosure 230 forms a covered or protectedarea 240 over reticle 200 and moving reticle stage base 210.Clean-enclosure 230 prevents particles from falling onto the surface ofreticle 200. Clean-enclosure 230 includes clean-enclosure frame (frame)250 and clean-enclosure cover (cover) 260.

In the illustrative embodiment described herein, frame 250 comprises ametal alloy such as an aluminum alloy. Alternatively, a plastic compoundor other material rigid enough to support cover 260 could be used. Frame250 rests securely on nonmoving reticle stage base 220 such that it iscentered around and encloses both reticle 200 and moving reticle stagebase 210. Frame 250 may be attached to nonmoving reticle stage base 220with adhesive materials. FIG. 2 shows frame 250 is a single, integralstructure. Alternatively, frame 250 could be formed from individualpieces fastened together by bolts, screws, adhesives, or other means.

The walls of frame 250 should lie completely outside the stepping rangeof moving reticle stage base 210. Other equipment, such as any reticlehandling equipment or particle detection equipment, for example, mayalso restrict the location of frame 250. Frame 250 may be rectangular asshown, however square or circular frames, for example, could be used ifotherwise compatible with the reticle, the moving reticle stage base,and the other requirements of the photolithographic equipment.

Cover 260 lies over protected area 240 and is attached to frame 250 withadhesive materials. Cover 260 comprises a flat, horizontal piece of anymaterial having adequate rigidity and thickness to lie completelyhorizontal over protected area 240. In order to properly align reticle200 and the semiconductor substrate, a source of alignment radiation maybe used. Cover 260 may comprise a material that is transmissive to thealignment radiation. For example, if the alignment radiation used is inthe visible range of wavelengths, cover 260 may comprise a material suchas quartz or glass. Alternatively, cover 260 could comprise a materialthat is not transmissive to the alignment radiation but which containsan alignment window or windows comprising a material transmissive to thealignment radiation.

Cover 260 contains a centrally located opening 280. By attaching border270 to cover 260, opening 280 is sealed by an exposure window 290.Border 270 surrounds exposure window 290, physically protecting exposurewindow 290 during its handling and attachment to cover 260. Exposurewindow 290 may comprise a thin film of silicon. Silicon, when formedinto a thin film with a thickness of 1000 angstroms, or less, is highlytransmissive to EUV radiation. FIG. 3 is a graph showing the dual-pathtransmission of EUV (13 nm) radiation through a thin film of silicon asa function of the film's thickness. Assuming that a dual-pathtransmission loss of 30 percent or less is acceptable, a correspondingsilicon film thickness of 1000 angstroms, or less, is preferred.

Alternatively, exposure window 290 could comprise another materialhaving a high exposed light transmission at the required radiationwavelength. The alternate materials could include typical pelliclemembranes 140 when used with non-EUV photolithography. For example, forphotolithography using ultra violet radiation with a wavelength ofapproximately 193 nm, exposure window 290 could comprise fused silica orcalcium fluoride (CaF). If it is required that clean-enclosure 230 bealigned with reticle 200 using radiation in the EUV range, cover 260 maycontain an alignment window or windows, comprising a thin film ofsilicon, to aid in alignment. If alignment at another wavelength isrequired, these alignment windows could be comprised of another materialhaving a high exposed light transmission at the required alignmentradiation wavelength.

In the illustrated embodiment, clean-enclosure 230 has a minimum overallheight as determined by a standoff distance t. As shown in FIG. 2, t isthe distance between exposure window 290 and the surface of reticle 200.Maintaining a minimum standoff distance prevents any particles that mayrest on exposure window 290 from focusing onto the photoresist layer ofthe semiconductor substrate. A typical minimum value for t is in therange of 6 to 10 mm. The overall height of clean-enclosure 230, beingequal to the height of frame 250 added to the thickness of cover 260,must be great enough to maintain such a minimum standoff distance. Anyadhesive materials used between frame 250 and cover 260, or betweenframe 250 and nomnoving reticle stage base 220, will contribute slightlyto the overall height of clean-enclosure 230. The height of frame 250 orthe thickness of cover 260 should be adjusted so that the overall heightis correct. The height of the clean-enclosure has no maximum theoreticallimit, but is usually restricted by the other requirements of thephotolithography equipment.

FIG. 4 is a top view illustrating the approximate shape and size of theradiation illumination reflected on the plane of reticle 200 based onone current EUVL design, “ringfield lithography.” In ringfieldlithography a stationary EUV radiation source, typically a synchrontronor a laser plasma source, is focused onto the reticle using a system ofreflective optics. The ringfield lithography technique balances loworder aberrations with higher order aberrations to create narrow annularregions of correction away from the optical axis of the system (regionsof constant radius, rotationally symmetric with respect to the axis).Consequentially, the shape of the corrected region, and the resultingimage field on the reticle surface as seen in FIG. 4, is an arcuatestrip rather than a straight strip.

FIG. 5 is a top view of the FIG. 2 clean-enclosure cover 260 andexposure window 290, surrounded by border 270, taken along a lineconnecting A-A′ of FIG. 2. The length l and approximate shape of coveropening 280 and corresponding exposure window 290 are dictated by thewidth of reticle 200 and the arcuate shape of the EUV radiation source,respectively. However, rectangular, square, or other off-axis fieldshapes are also possible and are contemplated. It is desirable that thewidth of exposure window 290 be as narrow as possible to minimize thestress placed on the silicon thin film material. Constraining the widthof exposure window 290 to a few millimeters is not limiting given thatreticle 200 is stepped on moving reticle stage base 210.

FIG. 6 is an enlarged view of the center portion of the FIG. 2 mask. Theradiation source reflects off reticle surface 200 at an incident angleØ. The standoff distance t must be a minimum height such that particlesthat may rest on exposure window 290 will not focus undesired patternsonto the photoresist layer on a semiconductor substrate. The standoffdistance of conventional pellicles is approximately 6 mm. To obtain adesired illumination width W_(b) on the reticle plane, the minimum widthW_(si) of exposure window 290 can be calculated from EQ. 1:

W _(si) =W _(b)+2t tanØ EQ. 1

where t is the standoff distance between exposure window 290 and thesurface of reticle 200, and Ø is the illumination incident angle. Forexample, when using an illumination incident angle Ø of 5 degrees and astandoff distance t of 10 mm, obtaining an illumination width W_(b) of 6mm on the reticle plane requires a minimum window width W_(si) of 9.5mm.

FIG. 7 illustrates the starting material used to form a silicon thinfilm exposure window in one embodiment of the present invention. Thestarting material consists of a silicon substrate 300. Next, a silicondioxide (SiO2) layer 310, as illustrated in FIG. 8, is formed on oneside of substrate 300 using a well-known deposition technique such aschemical vapor deposition.

Next, as shown in FIG. 9, a silicon thin film 320 is formed on thesurface of the silicon dioxide layer 310 using chemical vapor depositiontechniques. Silicon thin film 320 is formed with a thickness of 1000angstroms, or less, over substantially the entire surface of silicondioxide layer 310. As shown in FIG. 3, a silicon thin film with thisthickness is highly transmissive to EUV radiation.

Next, an opening is formed through the backside of substrate 300 for theexposure window. In this step, photoresist is applied over substrate 300and exposed to light and developed so that a desired opening is formedin photoresist mask 330 in accordance with the dimensions of theto-be-formed exposure window 290. As illustrated in FIG. 10, photoresistmask 330 covers the entire surface of substrate 300 except where theopening will be formed. The opening is formed through substrate 300 tosilicon dioxide layer 310, as shown in FIG. 11, when the part ofsubstrate 300 not covered by the photoresist is etched away using wetchemical or plasma etch processes. Photoresist mask 330 is then removedby rinsing in a chemical solution or by stripping using oxygenatedplasma.

As illustrated in FIG. 12, exposure window 290 is formed when theremaining silicon dioxide layer 310 is removed from the opening with awet etch process with high selectivity, for example, a selectivity of 50to 1, to the underlying silicon.

As illustrated in FIG. 13, some of the excess substrate 300 surroundingexposure window 290 may be retained as a border 270. Retaining a rigidborder around exposure window 290 has a number of advantages. Border 270physically supports and protects exposure window 290, allowing for easeof use and handling. Border 270 can readily be attached and detachedfrom cover 260 repeatedly without damaging exposure window 290. Thisfeature also allows clean-enclosure 230 to be used with a number ofdifferent exposure windows 290, comprising either silicon thin film oranother material, without necessitating the removal of clean-enclosure230 from nonmoving reticle stage base 220. If the size of the siliconsubstrate starting material used in forming exposure window 290 is largeenough, the excess substrate 300 bordering exposure window 290 can beused in place of all or part of cover 260. If border 270 is notdesirable, the excess substrate 300 surrounding exposure window 290 canbe physically removed and exposure window 290 can be directly attachedto cover 260 to seal opening 280.

The completed exposure window is used to seal the opening in the cover.The cover is secured to the frame and placed over a reflective reticle.During photolithography, the radiation is transmitted through theexposure window so that it reaches the reticle. The radiation reflectsoff the reticle and is again transmitted through the exposure window onits path to a semiconductor substrate. The exposure window providesradiation transmission to and from the reticle while the exposure windowand cover protect the reticle from particle contamination.

As described in the foregoing, the embodiments of the present inventionprovide a solution to the problem of protecting a photolithographicreticle to limit defects in manufactured semiconductor devices. Theinvention is particularly useful for EUVL. While the invention has beendescribed with reference to the structures and methods disclosed herein,it is not confined to the details set forth, rather, the invention isdefined by the scope of the following claims.

What is claimed is:
 1. A clean-enclosure comprising: a frame having twosides; a cover that is bonded to one side of the frame; and a windowwithin the cover, the window comprising a material transmissive at leastto an extreme ultraviolet (EUV) wavelength of photolithographicradiation.
 2. A clean-enclosure as in claim 1, wherein the covermaterial includes quartz.
 3. A clean-enclosure as in claim 1, whereinthe window comprises a thin film having a thickness in the range of 1 to1000 angstroms.
 4. A clean-enclosure as in claim 1, wherein the windowmaterial is selected from the group consisting of silicon, fused silica,and calcium fluoride.
 5. A clean-enclosure as in claim 1, wherein thewindow width is in the range of 1 to 20 millimeters.
 6. Aclean-enclosure as in claim 1, wherein the window is an arcuate shape.7. A clean-enclosure as in claim 1, wherein the window is a rectangularshape.
 8. A clean-enclosure as in claim 1, wherein the window issurrounded by a border.
 9. A clean-enclosure as in claim 1, wherein thewindow is formed within the material of the cover.
 10. A clean-enclosureas in claim 1, wherein the wavelength of photolithographic radiationused is in the range of 1 to 20 nanometers.
 11. The clean enclosure asin claim 1, wherein the frame comprises an aluminum alloy.
 12. The cleanenclosure as in claim 1, wherein the frame comprises a plastic compound.13. The clean enclosure as in claim 1, wherein the frame comprises asingle, integral structure.
 14. The clean enclosure as in claim 1,wherein the frame is attached to a nonmoving reticle stage base.
 15. Theclean enclosure as in claim 1, wherein the frame comprises individualpieces that are fastened together by means selected from bolts, screws,and adhesives.
 16. The clean enclosure as in claim 1, wherein the framecomprises a square shape.
 17. The clean enclosure as in claim 1, whereinthe frame comprises a circular shape.
 18. The clean enclosure as inclaim 1, further comprising a reticle, wherein a standoff distanceexists between the window and the reticle in a range from 6 mm to 10 mm.19. A method of forming a clean-enclosure window comprising: providing asilicon substrate having a first side and a second side; forming a firstlayer of material useful as an etch stop relative to silicon on thefirst side of the silicon substrate; forming a second layer over thefirst layer of material, the second layer being transmissive to at leastan extreme ultraviolet (EUV) wavelength of photolithographic radiation;forming an opening in the in the second side of the silicon substrate tothe first layer of material; and removing the first layer of material inthe opening so as to create a window from the remaining second layer ofmaterial.
 20. A method as in claim 19, wherein the first layer materialcomprises an oxide of silicon.
 21. A method as in claim 19, wherein thesecond layer comprises a thin film having a thickness in the range of 1to 1000 angstroms.
 22. A method as in claim 19, wherein the second layercomprises silicon.
 23. A method as in claim 19, wherein the window widthis in the range of 1 to 20 millimeters.
 24. A method as in claim 19,wherein the window is an arcuate shape.
 25. A method as in claim 19,wherein the window is a rectangular shape.
 26. A method as in claim 19,wherein the window is surrounded by a border.
 27. A method as in claim19, wherein the wavelength of photolithographic radiation used is in therange of 1 to 33 nanometers.
 28. The method as in claim 19, whereinforming an opening comprises forming an opening that has a minimum widthof about 9.5 mm.
 29. The method as in claim 19, wherein forming anopening comprises etching the silicon substrate with a wet chemical etchprocess.
 30. The method as in claim 19, wherein forming an openingcomprises etching the silicon substrate with a plasma etch process. 31.The method as in claim 19, wherein removing the first layer of materialin the opening comprises a wet etching process.
 32. The method as inclaim 19, wherein removing the first layer of material in the openingcomprises a wet etching process with an etch selectivity of about 50to
 1. 33. A method of protecting a photolithographic reticle comprising:mounting a reticle on a substrate; and enclosing the reticle in aclean-enclosure that is attached to the substrate, wherein the cleanenclosure includes a window comprising a layer of material that istransmissive to at least extreme ultraviolet (EUV) radiation.
 34. Amethod as in claim 33, wherein the reticle is a reflective reticle.